Camouflage nets are used to camouflage buildings as well as fixed and mobile military installations, such as, for example, vehicles, armored vehicles and the like. Such camouflage nets should not only provide camouflage from infrared cameras and thermal imaging detectors but also from radar detection. Camouflage should therefore be provided in the infrared, thermal imaging, extremely high and super high frequency radar beam range.
The purpose of the camouflage net is to prevent microwaves which impinge upon any object from being reflected by that object. It should also prevent identification by sensors operating in the infrared and thermal imaging range. This means that it should not be possible to recognize or identify the objects to be camouflaged by active video receivers in the 0.7-1.8 um range or by passive video receivers in the 3-5 um and 8-14 um range. Various camouflage nets are already known for this purpose.
In German Offenlegugsschrift 33 29 264 there is described a material which is absorbent in the microwave range and which is preferably effective in the 10 GHz range. However one disadvantage here is that it has to be applied to a metal base and is consequently unsuitable for flexible support materials.
In German Offenlegungsschrift 31 17 245 a metallized, roughened pile fabric is specified as the support material. However the metallized pile is present on one side only. This means that the desired reduced reflection of microwave radiation only occurs if the roughened side of the pile fabric faces the microwave transmitter. However the smooth reverse side possesses an almost undamped high reflective power. Another disadvantage is that this material displays very negative behavior in the thermal image range (heat absorption).
German Offenlegungsschrift 38 10 121 describes an open structure of polyester fibers as the support material for a camouflage net. However it has been shown that with this camouflage net the object beneath it is still visible as a thermal image. Also the high polymer content of the material with which the support material is coated, behaves in a very negative manner on the lattice structure or on the jacquard surface of the support material due to a very high level of heat absorption. One further disadvantage is that in the microwave range the polymer coating does not have an absorbent effect.
It is now known that in the atmospheric windows around 26-40 and 92-96 GHz natural objects, such as a grass and plants, behave like black body radiators with an emission level of almost one, whereas military objects, such as armored vehicles, lorries, etc. made of metal have an emission level of approximately zero and therefore a remission level of approximately one. Therefore the latter are ideal reflectors, with one part of the beam being directed, while one part provides a diffused reflection.
This means that with radiometric measurements from above the object to be camouflaged, e.g. a military object, reflects thermal radiation into the sky with a temperature of 30K at 35 GHz and 100K at 94 GHz, whereas the environment radiates as a black body radiator with ambient temperature.
The military object therefore behaves like a very cold target in a warm environment, with the temperature contrast being between 240K and 280K. In this way it can be detected as a cold body with a microwave radiometer. When the sky is overcast and it is still high enough for armored vehicles to be located with a passive microwave seeker head for the final phase guidance of shells and missiles.